Numerical Study On The Complex Heat Transfer Characteristics In The Waxy Crude Oil Tank

Most of the waxy crude oil is of high pour point,high viscosity and complex rheological properties,so it usually needs to be heated during production practice.Floating roof oil tank is the most important oil storage equipment.During tank storage process,the oil temperature will drop gradually due to the heat loss to the atmosphere.When oil temperature is lower than the thixotropy appearance temperature of the waxy crude oil,waxy crystal begins to appear.And when the waxy crystal proportion reaches2~3wt%of the whole waxy crude oil in the tank,gelation occurs,which will severely affect the oil production,even cause accident.Therefore,it is of great significances to study the waxy crude oil temperature drop process and grasp oil temperature drop characteristics accurately for designing the insulating structure for the floating roof oil tank scientifically,making the turnover schedule for the oil production reasonable,and reducing the production cost and saving energy.On the other hand,it is also very important to investigate the waxy crude oil melting characteristics in the tank when the oil is gel in the tank to understand the melting process,make reasonable melting schedule,save energy and reduce emission.Essentially,the waxy crude oil temperature drop and melting process in the floating roof oil tank are an unsteady natural convection involving phase change(waxy precipitation or melting),solid-liquid interaction,non-Newtonian behavior,fluid-solid coupling,as well as turbulence.Based on the changes of the oil state and rheological property during temperature drop process,and considering the coupling of the various materials of air layer,steel layer,insulating layer,soil layer and waxy crude oil in the tank,this essay establishes the physical and mathematical models for the waxy crude oil temperature drop and melting in the floating roof oil tank respectively,and then a coupled integrative numerical procedure for coupling the oil state,rheological property and various materials is developed.Moreover,with the further increase of turbulence in the model tank,1000m~3 floating roof oil tank,and 100,000m~3 double-plate floating roof oil tank,laminar model,Large Eddy Simulation and standard k-?model are developed to simulate the turbulent flow and heat transfer respectively.In addition,to improve the computational efficiency,unstructured grid and the immersed boundary method(IBM)are applied to discretize the irregular domain of the waxy crude oil in the model tank and 1000m~3 double-plate floating roof oil tank,respectively.Numerical simulation has been performed on the temperature drop process of the waxy crude oil in the floating roof oil tank,and the evolution of the waxy crude oil temperature field and velocity field in the tank are studied.Besides,the heat fluxes on the tank top,tank bottom and tank wall,as well as the gel oil growth on the tank top and tank bottom are analyzed respectively.In the model tank,the influences of the wax precipitation,non-Newtonian behavior and permeability constant of the wax crystal porous media on the temperature drop process are studied.What's more,the difference of the heat transfer characteristics between the 1000m~3 single-and double-plate floating roof oil tank are investigated.Furthermore,the difference between the laminar melting and turbulent melting in the tank are also studied.This research provides guidance for the floating roof oil tank designing,production schedule making,waxy crude oil storage,energy conservation and emission reduction.